The present invention is directed to integrating multiple optical elements on a wafer level. In particular, the present invention is directed to efficient creation of integrated multiple elements.
As the demand for smaller optical components to be used in a wider variety of applications increases, the ability to efficiently produce such optical elements also increases. In forming such integrated multiple optical elements at a mass production level, the need for accurate alignment increases. Further, such alignment is critical when integrating more than one optical element.
Integrated multiple optical elements are multiple optical elements stacked together along the z-axis, i.e., the direction of the light propagation. Thus, light travelling along the z-axis passes through the multiple elements sequentially. These elements are integrated such that further alignment of the elements with themselves is not needed, leaving only the integrated element to be aligned with a desired system, typically containing active elements.
Many optical systems require multiple optical elements. Such required multiple optical elements include multiple refractive elements, multiple diffractive elements and refractive/diffractive hybrid elements. Many of these multiple element systems were formed in the past by bonding individual elements together or bonding them individually to an alignment structure.
In bulk or macroscopic optics to be mounted in a machined alignment structure formed using a mechanical machining tools, the typical alignment precision that can be achieved is approximately 25-50 microns. To achieve a greater level of 15-25 microns, active alignment is required. Active alignment typically involves turning on a light source, e.g., a laser, and sequentially placing each optic down with uncured ultra-violet (UV) adhesive. Then each part is moved, usually with a translation stage, until the appropriate response from the laser is achieved. Then the part is held in place and the epoxy is cured with UV light, thereby mounting the element. This is done sequentially for each element in the system.
Alignment accuracies of less than 15 microns for individual elements can be achieved using active alignment, but such accuracies greatly increase the amount of time spent moving the element. This increase is further compounded when more than one optical element is to be aligned. Thus, such alignment accuracy is often impractical even using active alignment.
In many newer applications of optics, as in the optical head configuration set forth in commonly assigned co-pending application Ser. No. 08/727,837, which is hereby incorporated by reference, and the integrated beam shaper application noted above, there is a need to make optical systems composed of several micro-optical components and in which the tolerances needed are much tighter than can be achieved with conventional approaches. In addition to requiring tight tolerances, elements of lower cost are also demanded. The alignment tolerance needed may be 1 micron to 5 microns, which is very expensive to achieve with conventional methods.
To achieve greater alignment tolerances, passive alignment techniques have been used as set forth in U.S. Pat. No. 5,683,469 to Feldman entitled xe2x80x9cMicroelectronic Module Having Optical and Electrical Interconnectsxe2x80x9d. One such passive alignment technique is to place metal pads on the optics and on the laser and place solder between them and use self-alignment properties to achieve the alignment. When solder reflows, surface tension therein causes the parts to self-align. However, passive alignment has not been employed for wafer-to-wafer alignment. In particular, the high density of solder bumps required and the thickness and mass of the wafer make such alignment impractical.
Another problem in integrating multiple optical elements formed on separate wafers at a wafer level arises due to the dicing process for forming the individual integrated elements. The dicing process is messy due to the use of a dicing slurry. When single wafers are diced, the surfaces thereof may be cleaned to remove the dicing slurry. However, when the wafers are bonded together, the slurry enters the gap between the wafers. Removing the slurry from the gap formed between the wafers is quite difficult.
Integrated elements are also sometimes made by injection molding. With injection molding, plastic elements can be made having two molded elements located on opposite sides of a substrate. Multiple plastic elements can be made simultaneously with a multi-cavity injection molding tool.
Glass elements are also sometimes made by molding, as in U.S. Pat. No. 4,883,528 to Carpenter entitled xe2x80x9cApparatus for Molding Glass Optical Elementsxe2x80x9d. In this case, just as with plastic injection molding, multiple integrated elements are formed by molding two elements on opposite sides of a substrate. Glass molding however has disadvantages of being expensive to make tooling and limited in size that can be used.
To make optics inexpensive, replication techniques are typically used. In addition to plastic injection molding and glass molding discussed above, individual elements may also be embossed. An example of such embossing may be found in U.S. Pat. No. 5,597,613 to Galarneau entitled xe2x80x9cScale-up Process for Replicating Large Area Diffractive Optical Elementsxe2x80x9d. Replicated optics have not been used previously together with solder self-alignment techniques. For each replication method, many individual elements are generated as inexpensively as possible.
Such replication processes have not been used on a wafer level with subsequent dicing. This is primarily due to the stresses imposed on the embossed layer during dicing. When using embossing on a wafer level, unique problems, such as keeping the polymer which has been embossed sufficiently attached to the substrate, e.g., such that the alignment, especially critical on the small scale or when integrating more than one element, is not upset.
Further, these replication processes are not compatible with the wafer level photolithographic processes. In particular, replication processes do not attain the required alignment accuracies for photolithographic processing. Even if embossing was compatible with lithographic processing, it would be too expensive to pattern lithographically on one element at a time. Further, the chemical processing portion of lithographic processing would attack the embossing material.
Other problems in embossing onto plastic, as is conventionally done, and lithographic processing arise. In particular, the plastic is also attacked by the chemicals used in lithographic processing. Plastic also is too susceptible to warping due to thermal effects, which is detrimental to the alignment required during lithographic processing.
Considering the foregoing background, it is an object of the present invention to efficiently produce integrated multiple optical elements. Such efficient production is accomplished by forming the integrated multiple optical elements on a wafer level.
It is further an object of the present invention to address the problems arising when attempting to achieve such wafer level production of integrated multiple optical elements. These problems include ensuring accurate alignment, allowing precise dicing of the wafer to the constituent integrated multiple optical elements when more than one wafer is bonded together, and providing additional features for allowing easy incorporation of the integrated multiple optical element into an overall system for a desired application.
It is another object of the present invention to provide embossing which has sufficient alignment for use with photolithographic features and sufficient adhesion to withstand dicing.
These and other objects of the present invention will become more readily apparent from the detailed description given hereinafter. However, it should be understood that the detailed description gives specific examples, while indicating the preferred embodiments of the present invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.